Stereospecific β‐l‐Rhamnopyranosylation through an S_Ni‐Type Mechanism by Using Organoboron Reagents

Abstract

Stereospecific b-l-rhamnopyranosylations were conducted using a1 ,2-anhydro-l-rhamnopyranose donor and mono-ol or diol acceptors in the presence of ag lycosylacceptor-derived borinic or boronic ester.Reactions proceeded smoothly to providet he corresponding b-l-rhamnopyranosides (b-l-Rhap) with complete stereoselectivity in moderate to high yields without any further additives under mild conditions.M echanistic studies of the borinic ester mediated glycosylation using 13 Ck inetic isotope effect (KIE) measurements and DFT calculations were consistent with ac oncerted S N im echanism with an exploded transition state.I na ddition, the present glycosylation method was applied successfully to the synthesis of at risaccharide, a-l-Rhap-(1,2)-b-l-Rhap-(1,4)-Glcp,derived from Streptococcus pneumoniae serotypes 7B,7C, and 7D. Scheme 1. A) 2-Naphthylmethyl-mediated IAD toward complete stereoselective construction of b-l-rhamnoside. B) Stereoselective gold-catalyzed b-l-rhamnosylation with a2-alkynyl-4-nitro-benzoate donor. C) Stereospecific b-l-rhamnosylation using glycosyl-acceptor-derived borinic and boronic esters that proceedst hrough aS N i-type mechanism.

A synergistic Rh(I)/organoboron-catalysed site-selective carbohydrate functionalization that involves multiple stereocontrol

“…1d) 50 . Lately, the alternate utility of organoboron reagents as catalysts in glycofunctionalization is gaining traction [55][56][57][58][59][60][61][62][63] . Hence, assimilating organoboron catalysis synergistically with chiral Rh(I) catalysis would pave a direct route into hydronaphthalene glycoside motifs.…”

Site-Selective, Stereocontrolled Glycosylation Catalyzed by Bis-Thioureas

“…1b). The preference for (1,2)-glycosylation achieved with cat-1 is complementary to the (1,3)-selectivity obtained with current cyclic-adduct activation methods [16][17][18][19][20] . The dependence of site selectivity on catalyst absolute stereochemistry provided early evidence that selectivity originated from specific catalyst-substrate interactions, implying that further catalyst modifications might lead to enhanced selectivity.…”

“…There have been several important efforts to achieve non-enzymatic catalyst-controlled approaches to site selectivity in glycosylation reactions. One successful approach relies on the ability of certain Lewis acids to form cyclic covalent adducts with vicinal diols while selectively activating one of the transiently protected oxygens toward reaction [16][17][18][19][20] . In this manner, cis-1,2diols can be converted to cyclic adducts and induced to undergo selective glycosylations at the equatorial oxygen.…”

Stereoselective O‐Glycosylations by Pyrylium Salt Organocatalysis**

“…Stereoselective glycosylations have successfully been performed by internal delivery of the nucleophile (or “glycosyl acceptor”) [15, 16] . Toshima and Takahashi have recently introduced a method for 1,2‐ cis ‐glycosylations by boronic acid catalysis from the corresponding 1,2‐anhydro sugars [17–22] (Figure 1, middle) with excellent stereo‐ and regioselectivity. This glycosylation strategy relies on the formation of a boronic ester intermediate with the nucleophile and electrophile, resulting in a stereospecific glycosylation via an S N i reaction mechanism.…”